Not sure if this happens a lot, But I suspect it does. Probably more with the latter. You end up in an "intimate" situation with a brother or sister of your friend.

One of the best, most arousing (her past) stories my wife has confessed to me. Although, I can't let her know that it is.

My wife, although actually quite passive in bed. She isn't a wild thing, but she does get horny, and willing to let herself be enjoyed by a guy. Years ago, a friend of her adopted dad, was in town for a few days. It was summer, so she was on summer break, and her dad was at work during the days. He mother had passed away a few years before, and her sisters had moved out. So, just her and this older man at the house alone. He was older, divorced, lonely, horny and persuasive. She was.....naive, had a b/f, but they were not getting along. Luckily for this man....and me, the conditions were pretty much perfect. Some sob stories about being divorced from his mean wife. Some questions about her personal life, her b/f, and letting her know she was too good for her unkind b/f, and that she was attractive, and that if he were her age he would be begging to be with her, offering her an innocent back rub. Well, she does love her back rubbed still. And it easy to unbutton her pants from behind.

I ended up going out with a friends sister, he actually urged me to ask her out. After we started fucking he always found an excuse to show up at my place, he caught us one day when he just walked in without knocking. She didn't scream and run, just casually stood up and walked down the hall. I kicked him out, went back in the bedroom where she was playing with her pussy saying that was one of the hottest things she's ever done, and we fucked furiously while she called me his name! I asked her if she wanted to have a threesome and she said she'd think about it, but wanted to roleplay for now. I confronted him the next day and he admitted he's been trying to get into her pants for years, and thought she might be warming up to the idea. Long story short, we all got drunk one night and she let him watch us, then gave him a handjob. That never happened again, we broke up about 6 months later but I'm sure they went through with it. I asked him and he said he can neither confirm or deny my question...Wish I could have been there for that.